When drugs are to be injected into the human body, it is often essential that the dose to be injected is set with high precision, as a too high or too low dose can have undesired consequences. However, for persons being visually impaired or having poor dexterity it may be difficult to set the desired dose. Accordingly, it is desirable to provide an injection device with a preset mechanism which aids such persons in the process of setting a dose.
One such device is described in U.S. Pat. No. 6,228,067 disclosing an injection device having a movable expressing member for expressing an injection fluid from a fluid container. An actuating member is used for displacing the expressing member. Between the actuating member and the expressing member, a position-dependent connecting device is provided, which is disabled in a distal end position of the actuating member and is enabled in a position range adjacent to the distal end position of the actuating member, including a proximal end position. Thereby an injection device is obtained which is adapted to permit a patient to repeatedly administer the same constant injection dose. The preset dose is set by pulling the actuating member out until this movement is stopped by means of the connecting device. Subsequently the set dose is injected by pressing the actuating member. Thus, the preset mechanism has been applied to an injection device in which the dose setting as well as the injection is performed by means of linear movements. This is a disadvantage because setting and/or injecting a dose of one international unit corresponds to a very small movement of the actuating member, and it is therefore difficult to set and/or inject an accurate dose. Accordingly, it is also difficult to set the preset dose accurately and thereby to ensure subsequent accurate dosage. This is particularly disadvantageous because a too high or too low preset dose will result in a too high or too low dose being set each time the preset dose is set, and the accumulated dose over a longer period in time will therefore deviate substantially from what was intended.
U.S. Pat. No. 5,308,340 discloses a multiple dose injection pen comprising a follower which can be adjusted to any position along threads adapted to engage with internal threads of a dose knob. The principle function of the follower is to set a maximum allowable dose where the syringe is going to be used by persons who may have difficulty remembering the proper dosage. The follower limits the axial movement of the dose knob during setting of a dose, since the follower will abut a collar of the dose knob when the maximum dosage has been set. The follower and the collar are moved linearly into abutment.
EP 0 937 471 discloses a medication delivery pen having a repeat-dose feature that limits motion of the dose control mechanism using an adjustable repeat-dose stop on the dose knob. The pen comprises an adjustable dose stop which prevents axial movement of the dose knob beyond a set desired dose. When the desired dose has been set, a proximal face of the dose stop comes into contact with a distal face on a clicker, thereby preventing further axial movement of the dose knob. The faces are moved linearly into contact.